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HUMAN BRAIN T H E B O O K US_001_half_title.indd 1 01/08/18 12:56 PM US_002_003_title.indd 2 01/08/18 12:56 PM R I TA C A RT E R S U S A N A L D R I D G E M A R T Y N PA G E S T E V E PA R K E R CONSULTANTS Professor Chris Fr ith, Professor Uta Fr ith, and Dr. Melanie Shulman HUMAN BRAIN T H E B O O K US_002_003_title.indd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upa Rao ART EDITOR Sonakshi Singh MANAGING EDITOR Rohan Sinha MANAGING ART EDITOR Sudakshina Basu DTP DESIGNER Bimlesh Tiwary PICTURE RESEARCHER Sumedha Chopra PICTURE RESEARCH MANAGER Taiyaba Khatoon PREPRODUCTION MANAGER Balwant Singh PRODUCTION MANAGER Pankaj Sharma DK LONDON SENIOR EDITOR Peter Frances PROJECT EDITOR Ruth O’Rourke-Jones PROJECT ART EDITOR Francis Wong US EDITOR Jennette ElNaggar US EXECUTIVE EDITOR Lori Cates Hand MANAGING EDITOR Angeles Gavira Guerrero MANAGING ART EDITOR Michael Duffy JACKET DESIGN DEVELOPMENT MANAGER Sophia MTT PRODUCER, PREPRODUCTION Gilli; an Reid SENIOR PRODUCER Meskerem Berhane ASSOCIATE PUBLISHER Liz Wheeler ART DIRECTOR Karen Self DESIGN DIRECTOR Phil Ormerod PUBLISHING DIRECTOR Jonathan Metcalf US_004-005_contents.indd 4 01/08/18 12:56 PM MOVEMENT AND CONTROL REGULATION THE NEUROENDOCRINE SYSTEM PLANNING A MOVEMENT EXECUTING A MOVEMENT UNCONSCIOUS ACTION MIRROR NEURONS EMOTIONS AND FEELINGS THE EMOTIONAL BRAIN CONSCIOUS EMOTION DESIRE AND REWARD THE SOCIAL BRAIN SEX, LOVE, AND SURVIVAL EXPRESSION THE SELF AND OTHERS THE MORAL BRAIN LANGUAGE AND COMMUNICATION GESTURES AND BODY LANGUAGE THE ORIGINS OF LANGUAGE THE LANGUAGE AREAS A CONVERSATION READING AND WRITING MEMORY THE PRINCIPLES OF MEMORY THE MEMORY WEB LAYING DOWN A MEMORY RECALL AND RECOGNITION UNUSUAL MEMORY THINKING INTELLIGENCE CREATIVITY AND HUMOR BELIEF AND SUPERSTITION ILLUSIONS CONSCIOUSNESS WHAT IS CONSCIOUSNESS? LOCATING CONSCIOUSNESS ATTENTION AND CONSCIOUSNESS THE IDLING BRAIN ALTERING CONSCIOUSNESS SLEEP AND DREAMS TIME THE SELF AND CONSCIOUSNESS THE INDIVIDUAL BRAIN NATURE AND NURTURE INFLUENCING THE BRAIN PERSONALITY BRAIN MONITORING AND STIMULATION STRANGE BRAINS DEVELOPMENT AND AGING THE INFANT BRAIN CHILDHOOD AND ADOLESCENCE THE ADULT BRAIN THE AGING BRAIN THE BRAIN OF THE FUTURE DISEASES AND DISORDERS THE DISORDERED BRAIN DIRECTORY OF DISORDERS GLOSSARY INDEX ACKNOWLEDGMENTS 110 112 114 116 118 120 122 124 126 128 130 132 134 136 138 140 142 144 146 148 150 152 154 156 158 160 162 164 166 168 170 172 174 176 178 180 182 184 186 188 190 192 194 196 198 200 202 204 206 208 210 212 214 216 220 222 224 250 256 264 SENIOR EDITOR Peter Frances SENIOR ART EDITOR Maxine Lea PROJECT EDITORS Nathan Joyce, Ruth O’Rourke, Miezan van Zyl EDITORS Salima Hirani, Katie John, Rebecca Warren PROJECT ART EDITORS Alison Gardner, Siân Thomas, Francis Wong DESIGNER Riccie Janus EDITORIAL ASSISTANT Elizabeth Munsey INDEXER Hilary Bird PROOFREADER Polly Boyd PICTURE RESEARCHER Liz Moore JACKET DESIGNER Duncan Turner SENIOR PRODUCTION CONTROLLER Inderjit Bhullar PRODUCTION EDITOR Tony Phipps CREATIVE TECHNICAL SUPPORT Adam Brackenbury, John Goldsmid MANAGING EDITOR Sarah Larter SENIOR MANAGING ART EDITOR Phil Ormerod PUBLISHING MANAGER Liz Wheeler REFERENCE PUBLISHER Jonathan Metcalf ART DIRECTOR Bryn Walls ILLUSTRATORS Medi-Mation, Peter Bull Art Studio This American Edition, 2019 First American Edition, 2009 Published in the United States by DK Publishing 345 Hudson Street, New York, New York 10014 Copyright © 2009, 2014, 2019 Dorling Kindersley Limited DK, a Division of Penguin Random House LLC 19 20 21 22 23 10 9 8 7 6 5 4 3 2 1 001–306003–Jan/2019 All rights reserved. Without limiting the rights under the copyright reserved above, no part of this publication may be reproduced, stored in or introduced into a retrieval system, or transmitted, in any form, or by any means (electronic, mechanical, FIRST EDITION photocopying, recording, or otherwise), without the prior written permission of the copyright owner. Published in Great Britain by Dorling Kindersley Limited. A catalog record for this book is available from the Library of Congress. ISBN 978-1-4654-7954-9 DK books are available at special discounts when purchased in bulk for sales promotions, premiums, fund-raising, or educational use. For details, contact: DK Publishing Special Markets, 345 Hudson Street, New York, New York 10014 SpecialSales@dk.com The Human Brain Book provides information on a wide range of medical topics, and every effort has been made to ensure that the information in this book is accurate. The book is not a substitute for medical advice, however, and you are advised always to consult a doctor or other health professional on personal health matters. Printed in China A WORLD OF IDEAS: SEE ALL THERE IS TO KNOW www.dk.com US_004-005_contents.indd 5 01/08/18 12:56 PM NO ORDINARY ORGAN The human brain is like nothing else. As organs go, it is not especially prepossessing—3lb (1.4kg) or so of rounded, corrugated flesh with a consistency somewhere between jelly and cold butter. It doesn’t expand and shrink like the lungs, pump like the heart, or secrete visible material like the bladder. If you sliced off the top of someone’s head and peered inside, you wouldn’t see much happening at all. SEAT OF CONSCIOUSNESS Given this, it is perhaps not surprising that for centuries the contents of our skulls were regarded as relatively unimportant. When they mummified their dead, the ancient Egyptians scooped out the brains and threw them away, yet carefully preserved the heart. The Ancient Greek philospher, Aristotle, thought the brain was a radiator for cooling the blood. René Descartes, the French scientist, gave it a little more respect, concluding that it was a sort of antenna by which the spirit might commune with the body. It is only now that the full wonder of the brain is being realized. The most basic function of the brain is to keep the rest of the body alive. Among your brain’s 100 billion neurons, some regulate your breathing, heartbeat, and blood pressure and others control hunger, thirst, sex drive, and sleep cycle. In addition to this, the brain generates the emotions, perceptions, and thoughts that guide your behavior. Then it directs and executes your actions. Finally, it is responsible for the conscious awareness of the mind itself. THE DYNAMIC BRAIN Until about 100 years ago, the only evidence that brain and mind were connected was obtained from “natural experiments”—accidents in which head injuries created aberrations in their victims’ behavior. Dedicated physicians mapped out areas of the cerebral landscape by observing the subjects of such experiments while they were alive— then matching their deficits to the damaged areas of their brains. It was slow work because the scientists had to wait for their subjects to die before they could look at the physiological evidence. As a result, until the early 20th century, all that was known about the physical basis of the mind could have been contained in a single volume. Since then, scientific and technological advances have fueled a neuroscientific revolution. Powerful microscopes made it possible to look in detail at the brain’s intricate anatomy. A growing understanding of electricity allowed the dynamics of the brain to be recognized and then, with the advent of electroencephalography (EEG), to be observed and measured. Finally, the arrival of US_006-007_Intro.indd 6 01/08/18 5:23 PM functional brain imaging machines allowed scientists to look inside the living brain and see its mechanisms at work. In the last 20 years, positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and, most recently, magnetic encephalography (MEG) have among them produced an ever more detailed map of the brain’s functions. LIMITLESS LANDSCAPE Today we can point to the circuitry that keeps our vital processes going, the cells that produce our neurotransmitters, the synapses where signals leap from cell to cell, and the nerve fibers that convey pain or move our limbs. We know how our sense organs turn light rays and sounds waves into electrical signals, and we can trace the routes they follow to the specialized areas of cortex that respond to them. We know that such stimuli are weighed, valued, and turned into emotions by the amygdala—a tiny nugget of tissue that punches well above its weight. We can see the hippocampus retrieve a memory, or watch the prefrontal cortex make a moral judgment. We can recognize the nerve patterns associated with amusement, empathy— even the thrill of schadenfreude at the sight of an adversary suffering defeat. More than just a map, the picture emerging from imaging studies reveals the brain to be an astonishingly complex, sensitive system in which each part affects almost every other. “High level” cognition performed by the frontal lobes, for instance, feeds back to affect sensory experience—so what we see when we look at an object is shaped by expectation as well as by the effect of light hitting the retina. Conversely, the brain’s most sophisticated products can depend on its lowliest mechanisms. Intellectual judgments, for example, are driven by the body reactions that we feel as emotions, and consciousness can be snuffed out by damage to the humble brainstem. To confuse things further, the system doesn’t stop at the neck but extends to the tips of your toes. Some would argue it even goes beyond—to encompass other minds with which it interacts. Neuroscientific investigation of the brain is very much a work in progress and no one knows what the finished picture will look like. It may be that the brain is so complicated that it can never understand itself entirely. So this book cannot be taken as a full description of the brain. It is a single view, from bottom to top, of the human brain as we know it today—in all its beauty and complexity. Be amazed. US_006-007_Intro.indd 7 01/08/18 5:23 PM USING RATS The brains of rats are very similar to human brains. Until imaging techniques were developed, the only way scientists were able to look directly at brain tissue was by using the brains of rats and other non- human animals. IN V E S T IG A T IN G T H E B R A IN THE BRAIN IS THE LAST OF THE HUMAN ORGANS TO GIVE UP ITS SECRETS. FOR A LONG TIME, PEOPLE WERE NOT EVEN ABLE TO UNDERSTAND WHAT THE BRAIN IS FOR. THE DISCOVERY OF ITS ANATOMY, FUNCTIONS, AND PROCESSES HAS BEEN A LONG AND SLOW JOURNEY ACROSS THE MILLENNIA, AS HUMAN KNOWLEDGE ABOUT THIS MYSTERIOUS ORGAN HAS DEVELOPED AND ACCUMULATED. INVESTIGATING THE BRAIN EXPLORING THE BRAIN The brain is particularly difficult to investigate because its structures are minute and its processes cannot be seen with the naked eye. The problem is compounded by the fact that its most interesting product, consciousness, does not feel like a physical process, so there was no obvious reason for our distant ancestors to associate it with the brain. Nevertheless, over the centuries, philosophers and physicians built up an understanding of the brain and, in the last 25 years with the advent of brain-imaging techniques, neuroscientists have created a detailed map of what was once an entirely mysterious territory. 1700 BCE Egyptian papyrus gives a careful description of the brain, but Egyptians do not rate this organ highly; unlike other organs, it is removed and discarded before mummification, suggesting that it was not considered to be of any use in future incarnations. 2500 BCE Trepanation (boring holes into the skull) is a common surgical procedure across many cultures, possibly used for relieving brain disorders such as epilepsy, or for ritual or spiritual reasons. 450 BCE Early Greeks begin to recognize the brain as the seat of human sensation. 387 BCE The Greek philosopher Plato teaches at Athens; he believes the brain is the seat of mental processes. 335 BCE Greek philosopher Aristotle restates the ancient belief that the heart is the superior organ; the brain, a radiator to stop the body from overheating. 170 BCE Roman physician Galen theorizes that human moods and dispositions are due to the four “humors” (liquids that are held in the brain’s ventricles). The idea persists for more than 1,000 years. Galen’s anatomical descriptions, used by generations of physicians, were based mainly on work on monkeys and pigs. 1543 Andreas Vesalius, a European physician, publishes the first “modern” anatomy, with detailed drawings of the human brain. 1649 French philosopher René Descartes describes the brain as a hydraulic system that controls behavior. “Higher” mental functions are generated by a spiritual entity, however, which interacts with the body via the pineal gland. 1664 Oxford physiologist Thomas Willis publishes the first brain atlas, locating various functions in separate brain “modules.” 1774 German physician Franz Anton Mesmer introduces “animal magnetism,” later called hypnosis. 1791 Luigi Galvani, an Italian physicist, discovers the electrical basis of nervous activity by making frogs’ legs twitch. 4000 BCE Early Sumerian writing notes the euphoric effect of poppy seeds. DRAWING THE BRAIN BRAIN ATLAS PAPYRUS PLATO LUIGI GALVANI GALEN AT WORK RENÉ DESCARTES ARISTOTLE TREPANNING 1848 Phineas Gage has his brain pierced by an iron rod (see p.141). 1849 German physicist Hermann von Helmholtz measures the speed of nerve conduction and subsequently develops the idea that perception depends upon “unconscious inferences.” 8 4000 BCE 1500 1600 1700 18003000 BCE 2000 BCE 1000 BCE US_008-009_invest_brain.indd 8 01/08/18 5:23 PM IN V E S T IG A T IN G T H E B R A IN Circa 1900 Sigmund Freud abandons an early career in neurology to study psychodynamics. The success of Freudian psychoanalysis eclipsed physiological psychiatry for half a century. 1906 Santiago Ramón y Cajal describes how nerve cells communicate. 1906 Alois Alzheimer describes presenile degeneration (see p.231). 1909 Korbinian Brodmann describes 52 discrete cortical areas based on neural structure. These areas are still used today (see p.67). 1924 The first electroencephalograms are produced by Hans Berger. 1934 Portuguese neurologist Egas Moniz carries out the first leucotomy operations (later known as lobotomies, see p.11). He also invented angiography, one of the first techniques that allowed scientists to make images of the brain. 1957 W. Penfield and T. Rasmussen devise a motor and sensory homunculus (see pp.10 and 103). 1970–80 Brain scanning is developed: PET, SPECT, MRI, and MEG all emerge during this decade. 1973 Timothy Bliss and Terje Lomo describe long-term potentiation (see p.158). 1981 Roger Wolcott Sperry is awarded the Nobel Prize for his work on the different functions of the two brain hemispheres (see pp.11 and 205). 1983 Benjamin Libet writes on the timing of conscious volition (see p.11). 1862–74 Broca and Wernicke (see p.10) discover the two main language areas of the brain. 1850 Franz Joseph Gall founds phrenology (see p.10), which attributes different personality traits to specific areas of the head. NERVE CELLS IN RODENT HIPPOCAMPUS SIGMUND FREUD EGAS MONIZ ELECTROENCEPHALOGRAPHY NERVE CELLS EARLY MAGNETIC IMAGINGCORTICAL MAP 1889 Santiago Ramón y Cajal proposes that nerve cells are independent elements and the basic units of the brain in The Neuron Doctrine. He wins the Nobel Prize in 1906. 1953 Brenda Milner describes patient HM (see p.157), who suffers memory loss after hippocampal surgery. 1919 Irish neurologist Gordon Morgan Holmes localizes vision to the striate cortex (the primary visual cortex). 1859 Charles Darwin publishes On the Origin of Species. 1874 Carl Wernicke publishes on aphasia (language disorders after brain damage). 1914 British physiologist Henry Hallett Dale isolates acetylcholine, the first of the neuro- transmitters (see p.73) to be discovered. He wins the Nobel Prize in 1936. 1991 Mirror neurons are discovered by Giacomo Rizzolatti in Parma (see pp.11 and 122–23). 2013 The United States and European Union start human brain simulation projects. The Connectome, a global cooperative endeavor, delivers its first charts of the connections between neurons. Scientists were unable to find out much about the workings of the brain until relatively recently. The only way they were able to match functions such as sight, emotion, or speech to the locations in the brain in which they are controlled was to find a person in whom a faculty was disturbed due to injury, and then wait until they were dead in order to look at the location and extent of the brain damage. Otherwise, scientists could only guess at what was happening to the brain by observing people’s behavior. Today, modern imaging techniques such as functional MRI and EEG (see p.12) allow neuroscientists to see the electrical activity in the brain as a person carries out various tasks or thought processes. This allows them to link types of actions, emotions, and so on, to specific types of activity in the brain. The freedom to observe the brain that imaging techniques have afforded has allowed for an explosion of knowledge within neuroscience, and has deepened our understanding of the brain and how it works. THE ADVENT OF IMAGING TECHNIQUES MAGNETIC RESONANCE IMAGING Brain scans can reveal damaged tissue— the red area in the MRI scan above indicates damage caused by a stroke. ELECTRODES Neural activity can be measured by attaching electrodes to the scalp. These pick up electrical activity in the brain and transform it into a digital record. Electrode “cap” 1873 Italian scientist Camillo Golgi publishes the silver nitrate method, making it possible to see nerves in their entirety. He wins the Nobel Prize in 1906. 9 1900 2000 US_008-009_invest_brain.indd 9 01/08/18 5:23 PM L A N D M A R K S I N N E U R O S C IE N C E MOST OF THE KNOWLEDGE WE HAVE ABOUT THE BRAIN HAS BEEN GATHERED BY SLOW, PAINSTAKING RESEARCH INVOLVING LARGE TEAMS OF PEOPLE. HOWEVER, OCCASIONALLY THE HISTORY OF NEUROSCIENCE HAS BEEN PUNCTUATED BY DRAMATIC DISCOVERIES OR IDEAS, OFTEN ARISING FROM THE WORK OF A SINGLE SCIENTIST. SOME OF THESE SUBSEQUENTLY PROVED TO BE VALUABLE BREAKTHROUGHS WHILE OTHERS, ALTHOUGH INFLUENTIAL, PROVED TO BE DEAD ENDS. LANDMARKS IN NEUROSCIENCE PHRENOLOGY Franz Joseph Gall Gall thought that personality could be read by feeling the contours of the skull. He theorized that various faculties were localized in the brain and that the strongest were correspondingly large, making the skull bulge measurably. It was hugely popular in nineteenth-century America and Europe—nearly every town had a phrenology institute. Although nonsense, Gall’s idea that brain functions are localized has turned out to be largely true. Imaging research aimed at locating brain functions is often called “modern phrenology.” THE MAN WHO LOST HIMSELF Phineas Gage This polite, well-liked American railroad foreman changed dramatically, becoming “grossly profane,” after an accident destroyed part of his brain (see p.141). His case was the first to show that faculties such as social and moral judgment can be localized to the frontal lobes. FATEFUL INJURY This reconstruction of Phineas Gage’s skull shows how an iron rod damaged the frontal lobes of his brain. PHRENOLOGY HEAD Models such as this claimed to show the bulges on the skull that revealed a person’s character. Categories included “blandness” or “benevolence.” LANGUAGE AREAS Broca and Wernicke In 1861, French physician Paul Broca described a patient who he named “Tan,” as it was the only word “Tan” could say. When Tan died, Broca examined his brain and found damage to part of the left frontal cortex. This part of the brain became “Broca’s Area” (see p.148). In 1876, German neurologist Carl Wernicke found that damage to a different part of the brain (which became known as “Wernicke’s Area”) also caused language problems. These two scientists were the first to clearly define functional areas of the brain.CARL WERNICKEPAUL BROCA MAPPING THE BRAIN Wilder Penfield The first detailed maps of human brain function were made by Canadian brain surgeon Wilder Penfield. He worked with patients undergoing surgery to control epilepsy. While the brain was exposed, and the patient conscious, Penfield probed the cortex with an electrode and noted the responses of the patient as he touched each part. Penfield’s work was the first to reveal the role of the temporal lobe in recall and map the areas of the cortex that control movement and provide bodily sensations. EARLY BRAIN IMPLANT José Delgado Spanish neurologist Dr. José Delgado invented a brain implant that could be remotely controlled by radio waves. He found that animal and human behavior could be controlled by pressing a button. In a famous experiment, conducted in 1964, Delgado faced a charging bull, bringing it to a halt at his feet by activating the implant in its brain. In another, he put a device in the brain of a chimp that was bullying its mate. He put the control in the cage where the victim chimp used it to “turn off” the bully’s bad behavior. MODERN MAPPING Today advanced imaging (see above) allows neural activity to be matched to mental tasks. However, much of the basic map was established by Penfield half a century earlier. CANADIAN STAMPDELGADO AND THE BULL 1 0 US_010-011_breakthroughs.indd 10 02/08/18 2:32 PM CONSCIOUS DECISIONS Benjamin Libet A series of ingenious experiments by US neuroscientist Benjamin Libet (see p.191) in the early 1980s demonstrated that what we think are conscious “decisions” to act are actually just recognition of what the unconscious brain is already doing. Libet’s experiments have profound philosophical implications because, on the face of it, the results suggest that we do not have a conscious choice about what we do, and therefore cannot consider ourselves to have free will. SPLIT-BRAIN EXPERIMENTS Roger Sperry Neurobiologist Roger Sperry conducted the split-brain experiments (see p.204) on people whose brain hemispheres were surgically separated in the course of treatment for epilepsy. They showed that, under certain conditions, each hemisphere could hold different thoughts and intentions. This raised the profound question of whether a person has a single “self.” MAKING MEMORIES Henry G. Molaison In 1953, aged 27, “HM” underwent an operation in the US, to stem severe epilepsy. The surgeons, then unaware of the functions of the hippocampus, took out a large area of that part of his brain (see p.159). When he came round, he was unable to lay down new memories and remained so for the rest of his life. The tragic accident demonstrated the crucial role of the hippocampus in recall. FROZEN IN TIME Henry G. Molaison—generally known only as “HM” —was one of the most studied patients in the history of modern medicine. MIRROR NEURONS Mirror neurons (see pp.122–23) were discovered in 1991—by accident. A group of researchers in Italy, led by Giacomo Rizzolatti, were monitoring neural activity in the brains of monkeys as they made reaching movements. One day a researcher inadvertently mimicked the monkey’s movement while it was watching, and found that the neural activity in the monkey’s brain that sparked up in response to the sight was identical to the activity that occurred when the monkey made the action itself. Mirror neurons are thought by some to be the basis of theory of mind, mimicry, and empathy. MIMICKING MACAQUE Mirror neurons produce automatic mimicry by producing a similar state in an observer’s brain to the state of the person they are watching. INVESTIGATING FREE WILL LOBOTOMY The first lobotomies were performed in the 1890s, but they only took off in the 1930s when the Portuguese neurosurgeon Egas Moniz found that cutting the nerves from the frontal cortex to the thalamus relieved psychotic symptoms in some patients. Moniz’s work was picked up by US surgeon Walter Freeman, who invented the “ice pick lobotomy.” From 1936 until the 1950s, he advocated lobotomy to cure for a range of problems, and 40,000 to 50,000 patients were lobotomized. The operation was overused and is now thought abhorrent. However, in many cases it eased suffering: a follow-up of patients in the UK found 41 percent were “recovered” or “greatly improved,” 28 percent “minimally improved,” 25 percent had “no change,” 4 percent had died, and 2 percent were worse off. “ICE PICK” LOBOTOMY Walter Freeman, above, found he could perform a lobotomy under local anesthetic by hammering an ice pick above each eye of a patient and swishing the device back and forth like a windshield wiper.ICE PICK TREPANATION The practice of drilling holes in the head has been used since prehistoric times as a treatment for a vast array of illnesses. The modern equivalent, craniotomy, is carried out to relieve pressure within the skull. L A N D M A R K S IN N E U R O S C IE N C E ROGER SPERRY RECEIVES THE NOBEL PRIZE IN 1981 1 1 US_010-011_breakthroughs.indd 11 02/08/18 2:32 PM BRAIN IMAGING TECHNIQUES CAN BE DIVIDED INTO TWO DIFFERENT TYPES: ANATOMICAL IMAGING, WHICH GIVES INFORMATION ABOUT THE STRUCTURE OF THE BRAIN, AND FUNCTIONAL SCANNING, WHICH ALLOWS RESEARCHERS TO SEE HOW THE BRAIN WORKS. USED TOGETHER, THESE TECHNIQUES HAVE REVOLUTIONIZED NEUROSCIENCE. SCANNING THE BRAIN A WINDOW ON THE BRAIN The structure of the brain is well known, but until recently the way it created thoughts, emotions, and perceptions could only be guessed at. Imaging technology has now made it possible to look inside a living brain and see it at work. The brain works by generating tiny electrical charges. Functional imaging reveals which areas are most active. This may be done by measuring electrical activity directly (EEG), picking up magnetic fields created by electrical activity (MEG), or measuring metabolic side effects such as alterations in glucose absorption (PET) and blood flow (fMRI). PET SCANNER Positron emission tomography (PET) scanners detect signals from radioactive markers in tissues to show activity in the brain. BRAIN WAVES Electroencephalo- graphs (EEGs) show electrical activity caused by nerve cells firing. They record distinct “brain waves,” which reflect the speed of firing in different states of mind. FUNCTION The brain is composed of modules that are specialized to do specific things. Functional brain imaging is largely about identifying which ones are most concerned with doing what. This has allowed neuroscientists to build a detailed map of brain functions. We now know where perceptions, language, memory, emotion, and movement occur. By showing how various functions work together, imaging also gives us a glimpse into some of the most sophisticated aspects of human psychology. For example, observing a person’s brain making a decision, we see that apparently rational decisions are driven by the emotional brain. Imaging the brains of master chess players shows why expertise depends on practice. Watching the brain of a person seeing a frightened face shows that emotion is contagious. STRUCTURAL DETAILS These CT images show different tissues in detail. The image on the left shows the cerebellum and eyeballs in red, the bones in blue and green, and the sinuses and ear cavities in bright yellow. The image on the right shows a healthy brain (front at bottom). The black areas are the fluid-filled ventricles. 3-D BRAIN CT allows pictures of brains to be displayed in three dimensions, and “sliced” to reveal the inner workings. Here, the front right quarter of the brain’s coverings and surface are cut away to reveal the tissues beneath. ANATOMY The brain looks very different according to how it is viewed. Computed tomography (CT) imaging combines the use of a computer and fine X-rays to produce multiple “slices” of the body. It allows you to see normally obscured body tissues, such as the inside of the brain, from any angle or level, with the delicate inner structures thrown into clear relief. Artificial coloring of the areas further distinguishes one part from another. CT scans are purely structural: they show the form of the organ but not how it works. They are very good at showing contrast between soft tissues and bone, and are therefore useful in diagnosing tumors and blood clots. PET SCANS These scans involve injecting a volunteer with a radioactive marker that attaches to glucose in the brain. Areas of high activity (red) attract glucose for fuel. The marker dye shows which parts of the brain are firing. REAL-TIME ACTIVITY Magnetoencephalography (MEG) picks up magnetic traces of brain activity. It is poor at showing where activity occurs, but good at pinpointing timing. Here, a brain plans a finger movement, then 40 milliseconds later its activity shifts as the movement is made. MOVEMENT BEFORE MOVEMENT S C A N N IN G T H E B R A IN Sensory area Motor area Three- dimensional brain Computer- generated head Inner tissue 1 2 US_012-013_Scanning_brain.indd 12 01/08/18 5:23 PM COMBINED IMAGING Each type of imaging has its advantages. MRI is good on detail, for example, but is too slow to chart fast-moving events. EEG and MEG are fast but are not as good at pinpointing location. To get scans that show both fast processes and location, researchers use two or more methods to produce a combined image. Here (right), for example, high-resolution MRI, taking about 15 minutes to acquire, is combined with a low-resolution fMRI, which takes seconds to produce and shows the location of activity in the brain areas used in hearing language. The areas shift during a task like this that involves many aspects, and they have to work fast and in concert. The areas used in a task vary from person to person, so studies often combine data from volunteers to give an average. STUDYING LANGUAGE In most people, the main language areas of the brain are located in the left hemisphere, so this area shows greater activity when a person listens to spoken words. The right hemisphere is also required for complete hearing, and for distinguishing tone and rhythm. INNER STRUCTURES This MRI scan is set within an X-ray of the neck and skull. The MRI reveals the intricate folds of the brain tissue. FIBER DETAIL This diffusion tensor image shows another view of the nerve fibers. The green fibers link the various parts of the limbic system. The blue fibers run from the cerebellum, which joins onto the spine. The red fibers connect the two hemispheres. MOVEMENT FMRI is very good at localizing brain activity. In this image (bottom of brain at top), the red area shows activity in the part responsible for moving the right hand. Each side of the body is controlled by the opposite hemisphere of the brain. SLICED TOGETHER Here, a combined CT and MRI scan shows the surface folds of the brain. It also reveals the skull bones and the top vertebrae. MAGNETIC RESONANCE IMAGING Magnetic resonance imaging (MRI) provides a better contrast between tissue types than CT. Instead of using X-rays, it uses a powerful magnetic field, which causes hydrogen atoms in the body to realign. The nuclei of the atoms produce a magnetic field that is “read” by the scanner and turned into a three-dimensional computerized image. The brain is scanned at a rapid rate (typically once every 2–3 seconds) to produce “slices” similar to those in CT scans. Increases in neural activity cause changes in the blood flow, which alter the amount of oxygen in the area, producing a change in the magnetic signal. Functional MRI (fMRI) involves showing differing levels of electrical activity in the brain, overlaid on the anatomical details. NERVE PATHWAYS IN THE BRAIN A refinement of MRI called diffusion tensor imaging picks up the passage of water along nerve fibers. Here, the blue fibers run from top to bottom, the green from front to back, and the red between the two hemispheres. S C A N N IN G T H E B R A IN 1 3 US_012-013_Scanning_brain.indd 13 02/08/18 2:32 PM US_014-015_b_atlas_intro.indd 14 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN THE BRAIN IS THE MOST COMPLEX ORGAN IN THE BODY AND IS PROBABLY THE MOST COMPLEX SYSTEM KNOWN TO HUMANKIND. OUR BRAIN CONTAINS BILLIONS OF NEURONS THAT ARE CONSTANTLY SENDING SIGNALS TO EACH OTHER, AND IT IS THIS SIGNALING THAT CREATES OUR MINDS. WITH THE HELP OF MODERN SCANNING TECHNOLOGY, WE NOW KNOW ABOUT BRAIN STRUCTURE IN GREAT DETAIL. A JOURNEY THROUGH THE BRAIN In the nineteenth century, much was learned about the structure of the brain by removing it from the body after death. Knowledge of the workings of the living human brain could only be gained by studying people with damaged brains, for example Phineas Gage (see p.141), but the precise location of this damage could not be known while the patient was still alive. Everything changed with the invention of brain scanners at the end of the twentieth century. In the following pages, we shall undertake a journey through the brain of a healthy, 55-year- old man revealed by magnetic resonance imaging (MRI). In these images, we can see the many components of the brain. We are starting to understand the function of some of these, but we are only at the very beginning of this journey of understanding. The captions that accompany the scans indicate the most likely function of various brain regions. But these regions often have many functions, and these functions depend upon interactions with other brain regions. Most structures in the brain are paired, with identical counterparts in the left and right hemispheres, so structures identified in one hemisphere are mirrored in the opposite one. The scans themselves have been colored, so that the cerebrum appears in red, the cerebellum in light blue, and the brainstem in green. 1 5 US_014-015_b_atlas_intro.indd 15 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 1 THE FRONTAL-POLAR CORTEX The frontal-polar cortex is the most recently evolved part of the prefrontal cortex in the frontal lobe and is concerned with forward planning and the control of other brain regions. This slice, right at the front of the brain, also reveals other features of the skull, including the eyes, nasal cavity, maxillary sinus, and tongue. Frontal-polar cortex Eye Orbitofrontal gyrus Maxillary sinus Nasal cavity Frontal lobe 1 6 US_016-017_brain_atlas.indd 16 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN Frontal-polar cortex Olfactory bulb Orbitofrontal gyrus Nasal septum Optic nerve Tongue 2 THE FRONTAL LOBE The frontal lobe, of which the prefrontal cortex is the front part, is the largest of the brain’s lobes and the latest to evolve. The frontal lobe is devoted to the control of action—precise control of muscles at the back, high-level planning at the front. In this slice, the optic nerve can also be seen carrying visual information from the eye to the brain. 1 7 US_016-017_brain_atlas.indd 17 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN Optic nerve Nasal septum Tongue Masseter muscle Temporalis muscle Orbitofrontal gyrus Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus 3 THE CORTEX The cortex, which appears on these scans as yellow lines, is heavily folded, creating a large surface area. The major ingoing folds (sulci, singular sulcus) are used as landmarks to define brain regions. The bulges between the ingoing folds are known as gyri (singular, gyrus). The major components of the frontal lobe are the superior, middle, and inferior frontal gyri. 1 8 US_018-019_brain_atlas.indd 18 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN Nasal septum Tongue Masseter muscle Temporalis muscle Orbitofrontal gyrus Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus 4 THE ORBITOFRONTAL GYRI The orbitofrontal gyri, located at the bottom of the brain, receive signals about smell and taste. Like the rest of the prefrontal cortex, this area is concerned with predicting the future, but specializes in predictions about rewards and punishments and therefore emotions. This area is connected with the amygdala (see slice 9, p.24). 1 9 US_018-019_brain_atlas.indd 19 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 5 THE ANTERIOR CINGULATE CORTEX Here we see the beginning of the anterior cingulate cortex, which lies between the two hemispheres. This sits alongside the limbic system. It is involved in linking emotions to actions and predicting the consequences of actions. The back part of the anterior cingulate cortex has direct connections with the motor system. Orbitofrontal gyrus Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus Anterior cingulate cortex 2 0 US_020-021_brain_atlas.indd 20 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 6 THE TEMPORAL LOBES In this slice, the temporal lobes come into view for the first time. At the very front of the temporal lobes (the temporal poles), knowledge acquired from all the senses is combined, along with emotional tone. We can also see the lateral ventricles in the middle of the slice. These are parts of a system of fluid-filled spaces in the middle of the brain. Orbitofrontal gyrus Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus Anterior cingulate cortex Fusiform gyrus Middle temporal gyrus Lateral ventrical Temporal lobe 2 1 US_020-021_brain_atlas.indd 21 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 7 THE INSULA The insula is a fold of cortex hidden deep in the brain between the frontal and temporal lobes. Signals about the internal state of the body—such as heart rate, temperature, and pain—are received here. Also visible in this slice is the corpus callosum, the band of nerve fibers that joins the brain’s left and right hemispheres. Putamen Nucleus accumbens Fusiform gyrus Insula Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus Corpus callosum Lateral ventrical Head of caudate Anterior cingulate cortex Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Optic chiasm 2 2 US_022-023_brain_atlas.indd 22 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 8 THE BASAL GANGLIA Located in the middle of the brain, the basal ganglia include the caudate, putamen, and globus pallidus. Also known as nuclei, ganglia are clumps of gray matter (or nerve-cell bodies) surrounded by white matter. The basal ganglia are linked to the cortex, the thalamus, and the brainstem and are concerned with motor control and decision making. Putamen Fusiform gyrus Insula Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus Corpus callosum Lateral ventrical Head of caudate Anterior cingulate cortex Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Internal globus pallidus External globus pallidus Internal capsule Third ventrical Amygdala Hippocampus 2 3 US_022-023_brain_atlas.indd 23 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 9 THE AMYGDALA AND HIPPOCAMPUS This slice includes the amygdala and the front part of the hippocampus. Both structures lie in the inner part of the temporal lobe. The amygdala is involved in learning to approach or avoid things and hence with emotion. The hippocampus has a critical role in spatial navigation and memory of past experiences, including routes between places. Putamen Fusiform gyrus Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus Corpus callosum Lateral ventrical Head of caudate Anterior cingulate cortex Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Internal globus pallidus External globus pallidus Internal capsule Third ventrical Amygdala Hippocampus Insula 2 4 US_024-025_brain_atlas.indd 24 02/08/18 2:32 PM A J O U R N E Y T H R O U G H T H E B R A IN 10 BROCA’S AREA Here we approach the back of the frontal lobe. The bottom of the inferior frontal gyrus in the left hemisphere, just above the insula, contains Broca’s area, which has a critical role in speech and language. At the bottom of the slice, we see the front of the brainstem, the pons, which joins the brain to the spinal cord. Putamen Fusiform gyrus Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus Corpus callosum Lateral ventrical Head of caudate Anterior cingulate cortex Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Internal globus pallidus External globus pallidus Internal capsule Third ventrical Amygdala Hippocampus Spine Pons Ear Temporal horn of lateral ventrical Insula 2 5 US_024-025_brain_atlas.indd 25 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 11 THE THALAMUS This slice includes the thalamus, which lies between the cerebrum and the brainstem. A complex structure, the thalamus is made up of more than 20 nuclei (see p.60). The thalamus acts as a relay station, taking in information from all of the senses (except smell) and sending them on to different parts of the cerebral cortex. Putamen Fusiform gyrus Insula Inferior frontal gyrus Middle frontal gyrus Superior frontal gyrus Corpus callosum Lateral ventrical Precentral gyrus Anterior cingulate cortex Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Body of fornix Thalamus Third ventrical Hippocampus Pyramidal tract Ear Pons 2 6 US_026-027_brain_atlas.indd 26 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 12 THE BRAINSTEM The brainstem (in green) joins the brain to the spinal cord and contains a number of structures such as the pons. The brainstem has a special role in the control of basic body functions, including the control of heart rate and breathing. It also relays signals from the brain to the muscles and from senses in all parts of the body to the brain. Putamen Fusiform gyrus Insula Middle frontal gyrus Superior frontal gyrus Corpus callosum Lateral ventrical Precentral gyrus Anterior cingulate cortex Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Body of fornix Thalamus Third ventrical Hippocampus Pyramidal tract Pons Ear Cerebellum Temporal horn of lateral ventrical 2 7 US_026-027_brain_atlas.indd 27 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 13 THE PARIETAL LOBE The parietal lobe includes the supramarginal gyrus and the angular gyrus (see slices 14–20, pp.29–35). The parietal lobe integrates signals from many of the senses (including visual information that arrives via the dorsal route, see pp.84–85) to estimate the position of the body and the limbs in space. This information is critical when we reach for and grasp objects. Fusiform gyrus Insula Middle frontal gyrus Superior frontal gyrus Corpus callosum Lateral ventrical Postcentral gyrus Precentral gyrus Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Entorhinal cortex Ear Cerebellum Temporal horn of lateral ventrical Posterior cingulate cortex Pulvinar of the thalamus Parietal lobe 2 8 US_028-029_brain_atlas.indd 28 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 14 THE PRECENTRAL AND POSTCENTRAL GYRUS The last part of the frontal cortex is the precentral gyrus. This contains the motor strip, where different regions send signals to control different parts of the body. The immediately adjacent part of the parietal cortex (the postcentral gyrus) has a corresponding sensory strip, where sensory signals are received from different parts of the body. Fusiform gyrus Middle frontal gyrus Superior frontal gyrus Corpus callosum Lateral ventrical Postcentral gyrus Precentral gyrus Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Cerebellum Posterior cingulate cortex Supramarginal gyrus Vermis 2 9 US_028-029_brain_atlas.indd 29 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 15 THE PRIMARY AUDITORY CORTEX The primary auditory cortex, where signals from the ears reach the cortex via the thalamus, lies along the very top of the superior temporal gyrus, in the fissure between the temporal lobe and the parietal lobe. Adjacent to the primary auditory cortex is Wernicke’s area, where incoming sounds are turned into words. Lateral ventrical Postcentral gyrus Precentral gyrus Inferior temporal gyrus Middle temporal gyrus Superior temporal gyrus Cerebellum Supramarginal gyrus Fusiform gyrus Posterior cingulate cortex Vermis 3 0 US_030-031_brain_atlas.indd 30 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 16 THE FUSIFORM GYRUSThe inferior temporal gyrus and the fusiform gyrus at the bottom of the temporal lobe are two areas concerned with recognition of objects. Part of the fusiform gyrus, known as the face-recognition area, is specialized for recognizing faces. It not only identifies facial features but also scrutinizes them for meaning, so it plays an important part in social interaction. Lateral ventrical Postcentral gyrus Precentral gyrus Inferior temporal gyrus Middle temporal gyrus Cerebellum Supramarginal gyrus Vermis Posterior cingulate cortex Fusiform gyrus Occipital gyrus 3 1 US_030-031_brain_atlas.indd 31 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 17 THE CEREBELLUMThe cerebellum (colored light blue) is the highly convoluted “little brain” that sits at the back and below the main brain (also known as the cerebrum). The cerebellum is concerned with fine motor control and the timing of movements. There are many connections between the cerebellum and the motor cortex. Lateral ventrical Postcentral gyrus Inferior temporal gyrus Middle temporal gyrus Cerebellum Supramarginal gyrus Posterior cingulate cortex Occipital gyrus 3 2 US_032-033_brain_atlas.indd 32 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN 18 THE OCCIPITAL LOBEThe occipital lobe is concerned with vision. In the forward-most areas, signals from the primary visual cortex (see slice 20, p.35) are analyzed in terms of features such as shape and color. This information is then sent forward to the inferior temporal cortex (see slice 16, p.31), along a pathway called the ventral route, and used for object recognition. Lateral ventrical Postcentral gyrus Inferior temporal gyrus Angular gyrus Cerebellum Supramarginal gyrus Occipital gyrus Precuneus Occipital lobe 3 3 US_032-033_brain_atlas.indd 33 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN Angular gyrus Postcentral gyrus Lateral ventrical Cerebellum Superior parietal lobule Occipital gyrus Precuneus 19 THE PRECUNEUS AND THE POSTERIOR CINGULATE CORTEX The precuneus in the back part of the parietal lobe and posterior cingulate cortex (see slice 17, p.32) lie between the two hemispheres. These remain some of the more mysterious regions of the brain. They probably have a role in memory, especially memories about the self. 3 4 US_034-035_brain_atlas.indd 34 01/08/18 5:23 PM A J O U R N E Y T H R O U G H T H E B R A IN Angular gyrus Cerebellum Occipital gyrus Precuneus Superior parietal lobule Cuneus Primary visual cortex 20 THE PRIMARY VISUAL CORTEX The primary visual cortex is right at the back of the brain and lies mostly on the inside of the two hemispheres. This is the first point in the cortex where signals arrive from the eyes via the thalamus. These signals are retinotopically mapped—that is, a signal from a particular point on the retina is sent to a corresponding point on the primary visual cortex. 3 5 US_034-035_brain_atlas.indd 35 01/08/18 5:23 PM THE HUMAN BRAIN KEEPS US PRIMED TO RESPOND TO THE WORLD AROUND US. IT IS AT THE HUB OF A VAST AND COMPLEX COMMUNICATIONS NETWORK THAT CONSTANTLY SEEKS AND COLLECTS INFORMATION FROM THE REST OF THE BODY AND THE OUTSIDE WORLD. AS THE BRAIN INTERPRETS THIS INFORMATION, IT GENERATES EXPERIENCES—SIGHTS AND SOUNDS, EMOTIONS AND THOUGHTS. BUT ITS PRIMARY FUNCTION IS TO PRODUCE CHANGES IN THE BODY. THESE INCLUDE LIFE-SUSTAINING BASICS SUCH AS THE REGULAR CONTRACTIONS OF THE HEART THROUGH TO THE COMPLEX ACTIONS THAT CONSTITUTE BEHAVIOR. THE BRAIN AND THE BODY US_036-037_brainbody_op.indd 36 01/08/18 5:23 PM THE BRAIN AND THE BODY US_036-037_brainbody_op.indd 37 01/08/18 5:23 PM KEY FEATURES OF THE BRAIN FEATURE DESCRIPTION Processing information Sending signals Modules and connections Individuality Plasticity The brain registers a vast amount of information. However, only a very small amount of this is actually selected for processing to the point at which it enters our consciousness and can be reported. Experience that cannot be reported is not conscious. Unconscious brain processing nevertheless guides and sometimes initiates actions (see p.116 and p.191). The brain consists of about 1000 billon cells. Roughly 10 percent are specialized electrical cells called neurons, which send signals to one another; this signal transmission makes brain function different from any other bodily process. Although the signals are electrical, the mode of transmission between cells is chemical— the signals are passed on by substances called neurotransmitters. The brain is modular—different parts do different things. The modules are densely interconnected, however, and none works without the support of many others (and the rest of the body). Generally, lower-level functions, such as registering sensations, are strongly localized, but higher-level functions, such as memory and language, result from interconnections between brain areas. The basic “blueprint” of the brain is dictated by our genes. As with any other body feature, brains share a basic anatomy, but each one is also unique. Even identical twins have visibly different brains, right from the time they are born, because the brain is exquisitely sensitive to its environment. The differences between individual brains result in each person having a unique personality. Brain tissue can be “strengthened” and built up like a muscle, according to how much it is exercised. So, if a person learns and practices a skill, such as playing a musical instrument or doing mathematics, the part of the brain concerned with that task will grow physically bigger. It also becomes more efficient and enables the person to perform the task more skillfully. THE PRIMARY TASK OF THE BRAIN IS TO HELP MAINTAIN THE WHOLE BODY IN AN OPTIMAL STATE RELATIVE TO THE ENVIRONMENT, IN ORDER TO MAXIMIZE THE CHANCES OF SURVIVAL. THE BRAIN DOES THIS BY REGISTERING STIMULI AND THEN RESPONDING BY GENERATING ACTIONS. IN THE PROCESS, IT ALSO GENERATES SUBJECTIVE EXPERIENCE. BRAIN FUNCTIONS WHAT THE BRAIN DOES The brain receives a constant stream of information as electrical impulses from neurons in the sense organs. The first thing it does is determine whether the information warrants attention. If it is irrelevant or just confirmation that everything is staying the same, it is allowed to fade away and we are not conscious of it. But if it is novel or important, the brain amplifies the signals, causing them to be represented in various regions. If this activity is sustained for long enough, it will result in a conscious experience. In some cases, thoughts are taken one step further, and the brain instructs the body to act on them, by sending signals to the muscles to make them contract. HOW THE BRAIN DOES IT No one knows exactly how electrical activity turns into experience. That remains a famously hard problem, which has yet to be cracked (see p.179). However, much is now known about the brain processes that turn incoming information into the various components of subjective experience, such as thoughts or emotions. Much depends on where the information comes from. Each sense organ is specialized to deal with a different type of stimulus—the eyes are sensitive to light, the ears to sound waves, and so on. The sense organs respond to these stimuli in much the same way—they generate electrical signals, which are sent on for further processing. But the information from each organ is sent to a different part of the brain, and then processed along a different neural pathway. Where information is processed therefore determines what sort of experience it will generate.TH E B R A IN A N D T H E B O D Y I B R A IN F U N C T IO N S THE BRAIN AND BODY The brain and spinal cord constitute the central nervous system, which is the body’s main control center, responsible for coordinating all of the processes and movement in the body. ACTIONS Certain brain areas are specialized to produce body movement. Brainstem modules control automatic internal actions, such as the lung and chest movements needed for breathing, the beating of the heart, and the constriction or dilation of blood vessels to control blood pressure. In conscious activities, the primary motor cortex sends messages (via the cerebellum and basal ganglia) to the muscles of the limbs, trunk, and head to create gross movements. LANGUAGE Language involves both producing speech and analyzing what others say to understand the meaning. It depends on the brain’s ability to link objects with abstract symbols and then to convey the symbols—and thus the ideas they represent—to others via words. In addition to facilitating communication between people, language enables individuals to reflect on their own ideas. MEMORIES Some of the experiences we have change brain cells in such a way that the pattern of neural activity that produced the original experience can be replicated later in time. This process gives rise to recall, or memory, which enables us to use past experiences as a guide to how to behave in the present. F E E L I N G TA S T E 3 8 US_038-039_functions.indd 38 01/08/18 5:23 PM FEATURE FACT Structure Connectivity Growth Signaling speed Utilizing the whole brain Pain-free zone Regeneration The brain is highly compact. If you smoothed out all the wrinkles in the cortex, the brain would cover an area of about 2 1/2 square ft (2,300 square cm). The brain has around 100 billion neurons. There are more potential connections between the neurons than there are atoms in the universe. A fetus grows neurons at the rate of 250,000 a minute. A person is born with nearly all the neurons of an adult, but the neural networks are not mature yet. Information travels at different speeds within different types of neurons. Transmission speeds range from 3 to 330 feet/sec (1 to 100 meters/sec). The claim that we only use 10 percent of our brains is false—we use all of it. Some complex functions, such as memory, involve many areas at once. Brain tissue has no pain receptors, so despite the fact it registers pain from all parts of the body, it does not actually feel pain itself. You do not “lose” brain cells as you age, although some functions may decline. You can maintain the networks or even form new ones by exercising your brain. B R A IN F U N C T IO N S I T H E B R A IN A N D T H E B O D Y BRAIN FACTS THOUGHTS The brain uses sensations, perceptions, and emotions to generate action plans. Some of the plans give rise to internalized brain activity, or thoughts. “Inner speech,” for example, is actually generated by the motor areas, but has no visible sign. Some activity occurs in the hippocampus, which we experience as recollection. PERCEPTIONS Most of the time we are receiving information from many sensory areas at once, as with the combination of auditory and visual signals at a fireworks display. These signals may be communicated to association areas, which bind all of this information together. If these items of “bound” information become conscious, they form what is known as a multisensory perception. There is a great deal of current neuroscientific research on how the binding process formsa unified perception, because it is still not fully understood. SENSATIONS Information from the environment enters the brain via the different sense organs and is transmitted to specific areas of the cerebral cortex called the primary sensory areas. This information includes some input from the body itself. In the absence of external stimuli, the sensory areas continue to be active and are thought to generate the experiences that we know as dreams, hallucinations, and imagination. EMOTIONS Certain stimuli (including some thoughts and imaginings) cause changes in the body by activating areas in the limbic system, especially the amygdala. Conscious “feelings” occur when signals from the limbic system are sent on to “association areas” in the prefrontal cortex that support consciousness. During adolescence, the amygdala is relied heavily upon for processing emotional information, because the prefrontal cortex only matures when a person reaches their late 20s. F E E L I N G J U D G I N G T H I N K I N G P L A N N I N G S P AT I A L A W A R E N E S S R E C O G N I T I O N A R O U S A L C O O R D I N AT I O N V I S I O N S P E E C H C O M P R E H E N S I O N S O U N D S M E L L TA S T E M O V E M E N T T O U C H V I S U A L P R O C E S S I N G E M O T I O N M E M O R Y 3 9 US_038-039_functions.indd 39 01/08/18 5:23 PM TH E N E R V O U S SY ST E M TH E N ER VO U S SY ST EM IS T H E B O D Y’ S M A JO R C O M M U N IC A TI O N A N D C O N TR O L N ET W O RK . D A TA , I N T H E FO RM O F E LE C TR IC A L SI G N A LS , I S R EL A YE D C O N ST A N TL Y FR O M T H E SE N SE O RG A N S TO A N D F RO M T H E B RA IN , T H RO U G H C O M PL EX N ET W O RK S O F N EU RO N S A N D O N A T IM ES C A LE M EA SU R ED IN M IL LI SE C O N D S. A lt ho ug h it is a s in gl e, u ni fie d co m m un ic at io ns n et w or k, th e ne rv ou s sy st em c on si st s of t hr ee a na to m ic al a nd fu nc ti on al s ub di vi si on s. T he c en tr al n er vo us s ys te m (C N S) is t he c oo rd in at in g sy st em f or t he b od y. I t co m pr is es t he b ra in a nd s pi na l c or d, w hi ch a re su rr ou nd ed a nd p ro te ct ed b y th e sk ul l a nd v er te br al co lu m n re sp ec ti ve ly . T he p er ip he ra l n er vo us s ys te m (P N S) is a c om pl ex n et w or k of n er ve s ex te nd in g ac ro ss th e bo dy , b ra nc hi ng o ut f ro m 1 2 pa ir s of c ra ni al n er ve s or ig in at in g in t he b ra in a nd 3 1 pa ir s of s pi na l n er ve s em an at in g fr om t he s pi na l c or d. I t re la ys in fo rm at io n be tw ee n th e bo dy a nd t he b ra in in t he f or m o f ne rv e im pu ls es . I t ha s an a ff er en t di vi si on ( th ro ug h w hi ch m es sa ge s ar e se nt t o th e br ai n) a nd a n ef fe re nt d iv is io n (w hi ch c ar ri es m es sa ge s fr om t he b ra in t o th e bo dy ). F in al ly , t he re is t he a ut on om ic n er vo us s ys te m ( A N S) , w hi ch s ha re s so m e ne rv e st ru ct ur es w it h bo th t he C N S an d P N S. I t fu nc ti on s “a ut om at ic al ly ” w it ho ut c on sc io us aw ar en es s, c on tr ol lin g ba si c fu nc ti on s, s uc h as b od y te m pe ra tu re , b lo od p re ss ur e, a nd h ea rt r at e. Se ns or y in pu t tr av el s qu ic kl y fr om re ce pt or p oi nt s th ro ug ho ut t he b od y vi a th e af fe re nt n et w or k of t he P N S to t he br ai n, w hi ch p ro ce ss es , c oo rd in at es , a nd in te rp re ts t he d at a in ju st f ra ct io ns o f a se co nd . T he b ra in m ak es a n ex ec ut iv e de ci si on t ha t is c on ve ye d vi a th e ef fe re nt di vi si on o f th e P N S to m us cl es , w hi ch ta ke t he a ct io n ne ed ed t o re sp on d to en vi ro nm en ta l c ha ng e ra pi dl y. T H E B R A IN A N D T H E B O D Y U ln ar n er ve V ag us n er ve P hr en ic n er ve La te ra l p ec to ra l n er ve La te ra l c ut an eo us b ra nc he s o f in te rc o st al n er ve s M us cu lo cu ta ne o us n er ve M ed ia l c ut an eo us b ra nc he s o f in te rc o st al n er ve s D o rs al b ra nc he s o f in te rc o st al ne rv es Su b co st al n er ve Ili o in g ui na l n er ve Ili o hy p o g as tr ic n er ve In te rc o st al n er ve s O b tu ra to r ne rv e Fi lu m t er m in al e G lu te al ne rv e B ra in Fa ci al n er ve B ra ch ia l p le xu s Fe m o ra l n er ve D el to id n er ve Su p ra cl av ic ul ar n er ve A xi lla ry n er ve M ed ia n ne rv e R ad ia l n er ve Sp in al g an g lio n Sp in al co rd 4 0 US_040-041_Nervous_system.indd 40 02/08/18 2:32 PM C o m m o n p al m ar d ig it al n er ve D ee p b ra nc h o f ul na r ne rv e La te ra l p la nt ar n er ve In te rm ed ia te d o rs al cu ta ne o us n er ve In fr ap at el la r b ra nc h o f sa p he no us n er ve In te ro ss eo us n er ve C ut an eo us b ra nc h o f sa p he no us n er ve Sa p he no us n er ve M ed ia l p la nt ar n er ve M us cu la r b ra nc h o f fe m o ra l n er ve A nt er io r cu ta ne o us b ra nc he s o f fe m o ra l n er ve M us cu la r b ra nc he s o f sc ia ti c ne rv e C o m m o n p er o ne al n er ve M ed ia l d o rs al cu ta ne o us n er ve D ee p p er o ne al n er ve M us cu la r b ra nc h o f ti b ia l n er ve Su p er fi ci al p er o ne al n er ve Ti b ia l n er ve Sc ia ti c ne rv e P ud en d al n er ve C E LL S — N E U R O N S N eu ro ns a re th e b as ic u ni ts of th e C N S. T he y tr an sm it el ec tr ic al s ig na ls , p ro ce ss th e d at a, a nd c om m un ic at e w ith ea ch o th er v ia s yn ap se s. N E T W O R K S N eu ra l n et w o rk s co ns is t o f th o us an d s o f n eu ro ns a nd th e co nn ec tio ns b et w ee n th em (s yn ap se s) . O R G A N — TH E B R A IN Th e ce nt ra l o rg an o f t he C N S, th e b ra in is a c o m p le x, in te g ra te d c o lle ct io n o f t is su es th at c o nt ro ls t he fu nc tio ns o f t he h um an b o d y. SY ST E M — TH E C E N TR A L N E R V O U S SY ST E M Th e b ra in a nd t he s p in al c o rd to g et he r m ak e up t he C N S. TI SS U E S — N U C LE I Th es e ar e g ro up s o f n eu ro ns (n uc le i) th at w o rk to g et he r t o p er fo rm s p ec ia liz ed fu nc tio ns . M O LE C U LE S Th es e ar e th e sm al le st re co gn iz ed u ni t, co m pr is in g tw o or m or e at om s. A ll th e bo dy ’s ce lls c on ta in w or ki ng p ar ts m ad e of m ill io ns o f t he m . In cr ea si ng ly , t he in te ra ct io n b et w ee n b ra in a nd b o d y is b ei ng un d er st o o d in m uc h fin er d et ai l. Th e o rg an iz at io n o f t he ne rv o us s ys te m (a nd fo r th at m at te r, al l t he o th er s ys te m s o f th e b o d y, s uc h as t he c ar d io va sc ul ar a nd e nd o cr in e sy st em s) ca n b e co ns id er ed a t va ri o us d iff er en t fu nc ti o na l l ev el s, fr o m th e en ti re s ys te m d o w n to in d iv id ua l c el ls , t he b as ic u ni t o f al l l iv in g t hi ng s. T he c ha rt b el o w s ho w s si x o f t he se le ve ls an d t he ir fe at ur es . W hi le it is p o ss ib le t o v ie w o rg an s w it h th e na ke d e ye , t is su es , n et w o rk s, c el ls , a nd m o le cu le s al l ha ve t o b e vi ew ed w it h th e ai d o f a m ic ro sc o p e. T H E B R A IN A N D T H E B O D Y 4 1 US_040-041_Nervous_system.indd 41 02/08/18 2:32 PM Meninges Three layers of connective tissues that protect spinal cord; cerebrospinal fluid fills space under middle layer T H E B R A IN A N D T H E B O D Y I T H E B R A IN A N D T H E N E R V O U S S Y S T E M THE BRAIN SITS AT THE TOP OF THE BODY, DIRECTING AND COORDINATING ALL ACTION AND ACTIVITY THROUGHOUT ITS ENTIRETY. IT DOES SO VIA THE SPINAL CORD, AND THE NERVES THAT STEM FROM IT AT VARIOUS POINTS ALONG ITS LENGTH AND BRANCH OUT INTO A NETWORK THAT SPANS THE WHOLE BODY. THE BRAIN AND THE NERVOUS SYSTEM THE SPINAL CORD The spinal cord carries information to and from the brain and all parts of the body except the head, which is served by the cranial nerves. The signals that travel along the spinal cord are known as nerve impulses. The cord itself comprises a bundle of nerve fibers, which are the long projections of nerve cells. They extend from the base of the brain to the lower region of the spine. The cord is roughly the width of a pencil, tapering at its base to a narrow bunch of fibers. Data from the sensory organs in different parts of the body is collected via the spinal nerves and transmitted along the spinal cord to the brain. The spinal cord also sends motor information, such as movement commands, from the brain out to the body, again transmitted via the spinal nerve network. EXTENT OF THE SPINAL CORD The spinal cord extends from the brainstem down to the first lumbar vertebra, where it forms a filament, known as the filum terminale, that extends to the coccyx. SPINAL CORD ANATOMY The core of the spinal cord is gray matter, which is composed of nerve cells (neurons). The outer layer of white matter insulates the long fibers (axons) that extend from the nerve cells. Subarachnoid space Spinal nerve Carries both sensory and motor information between brain and body Nerve fibers Bundles of nerve fibers carry signals to and from spinal cord and specific areas of the brain White matter Gray matter Sensory root ganglion Cluster of nerve cell bodies on each spinal nerve; partially processes incoming signals Motor nerve rootlet Individual nerve fiber that emerges from front of spinal cord; carries signals to muscles FRONT OF BODY Sensory nerve root Nerve splits into rootlets that enter spinal cord at rear, carrying incoming signals about touch sensations to brain Pia mater Arachnoid Dura mater Anterior fissure Deep groove along front of spinal cord cortex Central canal Filled with cerebrospinal fluid, which provides nourishment REAR OF BODY Spinal nerve root Spinal nerve Spinal cord Sacral region Six pairs of sacral nerves connect to legs, feet, and anal and genital areas Cervical region Eight pairs of cervical nerves serve chest, head, neck, shoulders, arms, and hands Lumbar region Five pairs of lumbar nerves form network to serve lower abdomen, thighs, and legs Thoracic region 12 pairs of thoracic nerves connect to back and abdominal muscles and intercostal muscles HOW SPINAL NERVES ATTACH There are gaps in the vertebrae of the backbone through which spinal nerves enter the spinal cord. The nerves divide into spinal nerve roots, each made up of tiny rootlets that enter the back and front parts of the cord.Vertebra Spinal nerves contain a special fiber, the dorsal root, that sends sensory information from the skin to the brain. All but one pair of spinal nerves serves a specific area of the body, or dermatome. Nerve fibers in contact with skin receptors join up along the network of fibers in one dermatome to form the relevant dorsal root, which enters the spinal cord and conveys sensory impulses from that dermatome to the brain. SPINAL NERVES There are 31 pairs of spinal nerves. These branch out from the spinal cord, dividing and subdividing to form a network connecting the spinal cord to every part of the body. The spinal nerves carry information from receptors around the body to the spinal cord. From here the information passes to the brain for processing. Spinal nerves also transmit motor information from the brain to the body’s muscles and glands so that the brain’s instructions can be carried out swiftly. SPINAL REGIONS Each of the 31 pairs of nerves belong to one of four spinal regions— cervical, thoracic, lumbar, or sacral. MAP OF DERMATOMES This map shows the 30 dermatomes of the body. Each zone is served by a corresponding pair of spinal nerves. lumbar region brainstem coccyx spinal cord filum terminale DERMATOMES 4 2 US_042-043_nervous_system.indd 42 02/08/18 2:32 PM T H E B R A IN A N D T H E N E R V O U S S Y S T E M I T H E B R A IN A N D T H E B O D Y CRANIAL NERVES There are 12 pairs of cranial nerves that are linked directly to the brain and do not enter the spinal cord. They allow sensory information to pass from the organs of the head, such as the eyes and ears, to the brain and also convey motor information from the brain to these organs—for example, directions for moving the mouth and lips in speech. The cranial nerves are named for the body part they serve, such as the optic nerve for the eyes, and are also assigned Roman numerals, following anatomical convention. Spinal accessory nerve (XI, mixed) Motor functions responsible for muscles and movements of head, neck, and shoulders; also stimulates muscles of larynx and pharynx, which are involved in swallowing; sensory functions unknown CRANIAL NERVE CONNECTIONS The cranial nerves I and II connect to the cerebrum, while cranial nerves III to XII connect to the brainstem. The fibers of sensory cranial nerves each project from a cell body that is located outside the brain itself, in sensory ganglia or elsewhere along the trunks of sensory nerves. Vagus nerve (X, mixed) Longest and most branched of all cranial nerves, with autonomic, sensory, and motor fibers; serves lower part of head, throat, neck, chest, and abdomen, and plays role in many functions, including swallowing, breathing, heartbeat, and production of stomach acid Facial nerve (VII, mixed) Sensory fibers collect information from taste buds at front two-thirds of tongue; motor fibers are predominantly responsible for muscle movements controlling facial expression and also function of salivary gland and lacrimal gland, which secretes tears and lubricates the surface of the eye and conjunctiva of the eyelid Trigeminal nerve (V, two sensory and one mixed branch) Ophthalmic and maxillary branches of this nerve convey signals from eyes, teeth, and face, and other sensory fibers carry impulses from lower jaw; motor fibers control muscles involved with chewing Optic nerve (II, sensory) Visual information from retina is conveyed to brain by optic nerve at back of eye; optic nerves from both eyes meet at point known as optic chiasm, then signals from both visual fields are sent to opposite sides of brain Glossopharyngeal and hypoglossal nerves (IX, XII, both mixed) Motor fibers of these nerves control most of the muscles involved with tongue movement and swallowing; sensory fibers convey information on taste, touch, and temperature from tongue and pharynx and can trigger gag reflex if stimulated Vestibulocochlear nerve (VIII, sensory) Vestibular branch of this nerve collects information from inner ear about head orientation and balance; cochlear branch is concerned with sound and hearing signals from ear Oculomotor, trochlear, and abducens nerves (III, IV, VI, motor) Three nerves regulating voluntary movements of eye muscles, allowing movement of eyeball and eyelids; oculomotor nerve also allows for pupil constriction Olfactory nerve (I, sensory) Smell molecules in nasal cavity trigger nerve impulses that pass along this nerve to olfactory bulb, then on to limbic areas (see pp.64–65) of brain II VII XI X VIII IX XII III IV VI I V 4 3 US_042-043_nervous_system.indd 43 01/08/18 5:23 PM THE BRAIN ACCOUNTS FOR AROUND 2 PERCENT OF TOTAL BODY WEIGHT, BUT CONSUMES A DISPROPORTIONATE AMOUNT OF FUEL TO SUPPORT ITS MANY ACTIVITIES. IT HAS SEVERAL FORMS OF PROTECTION—THE LAYERS OF MEMBRANE SURROUNDING IT, A BONY SKULL, AND FLUID PRODUCED IN ITS CHAMBERS (VENTRICLES) TO ABSORB THE IMPACT OF SHOCKS. BRAIN SIZE, ENERGY USE, AND PROTECTION WEIGHT AND VOLUME The average adult human brain weighs about 31/4 lb (1.5kg). Its volume and shape are similar to those of an average-sized cauliflower, and the consistency of its tissues is similar to stiff jelly. The size of a person’s brain bears little relation to his or her intelligence, and every brain, whatever its weight and volume, has roughly the same number of neurons and synapses. After the age of 20 or so, brain mass decreases by about 1/32 oz (1g) per year. New neurons are made throughout life, but not enough to replace those that die off with age. This is generally no cause for concern, as there are plenty of neurons left to carry out the brain’s functions. COMPOSITION OF THE BRAIN The brain consists mainly of water, which occurs in the cytoplasm of neurons and glial cells, as well as being a major constituent of blood. The brain is also rich in lipids—fatty molecules that make up cell membranes. BRAIN WEIGHT The brain’s weight increases from birth and reaches its maximum during adolescence. The number of neurons is fixed in infancy but, as the body grows, they grow in size and form new connections. The male brain is consistently heavier than the female brain from birth. AGE (IN YEARS) BRAIN WEIGHT AND BODY WEIGHT This graph shows brain weight as a percentage of total body weight over the course of a lifetime. Proportionally, a baby’s brain is around six times larger than an adult’s. Despite being lighter than the male brain overall, the female brain after the age of 13 is actually heavier than the male brain as a proportion of the entire body’s weight. B R A IN W E IG H T A S A % O F B O D Y W E IG H T W E IG H T O F B R A IN ( K G ) MALE FEMALE KEY % O F IN TR A C R A N IA L C O N TE N T % C O M P O SI TI O N O F TH E B R A IN 1.5 1 0.5 0 0.25 0.75 1.25 0 10 20 30 40 50 60 70 80 8 4 12 0 2 6 10 14 0 10 20 30 40 50 60 70 80 10% blood 10–12% lipids (fatty molecules) 10% CSF 8% protein 2% soluble organic substances 1% carbohydrate 1% inorganic salts 77–78% water cerebellar degeneration A recent study linked alcohol consumption to brain shrinkage. Participants disclosed their drinking habits and MRI scanning was used to measure each person’s ratio of brain volume to skull size. It was found that abstainers had greater brain volumes than former drinkers, light drinkers, moderate drinkers, or heavy drinkers. On average, abstainers had 1.6 percent greater brain volume than heavy drinkers. Interestingly, the effects were most marked among elderly women. In another study, participants between the ages of 60 and 79 took up either regular aerobic exercise or toning and stretching exercises for six months. MRI scans of each participant taken both before and after the six-month period showed an increase in the brain volumes of those doing aerobic exercise, suggesting that aerobic exercise can help maintain the health of the brain in older adults. BRAIN VOLUME AND LIFESTYLE BRAIN OF A NORMAL MALE BRAIN OF AN ALCOHOLIC ALCOHOLISM AND BRAIN ATROPHY Alcoholism can lead to cerebellar degeneration as shown above. The low quality of the scan was due to the man’s withdrawal symptoms, preventing him from sitting still. The brain is housed within the intracranial cavity, so measurements of the skull effectively relate to the size of the brain. The actual length, width, and height of an individual human brain can be measured using MRI scanning. There is considerable variation in the size of the adult human brain, but the average dimensions are given against the diagrams below. Bear in mind that, because of the numerous complex folds within the cerebrum, the brain has a much larger surface area than is apparent from its overall shape. LENGTH, WIDTH, AND HEIGHT 51/2 IN (140MM) 3 1/2 IN (93M M ) 61/2 IN (167MM) LEFT HEMISPHERE INTRACRANIAL CONTENT Brain tissue comprises gray and white matter, which consist of neurons and supporting glial cells respectively. A series of ventricles is filled with cerebrospinal fluid (CSF) and the brain is also richly supplied with blood vessels. FRONT T H E B R A IN A N D T H E B O D Y I B R A IN S IZ E , E N E R G Y U S E , A N D P R O T E C T IO N 80% brain tissue AGE (IN YEARS) 4 4 US_044-045_size_weight.indd 44 02/08/18 2:32 PM OXYGEN AND GLUCOSE SUPPLY Glucose is the brain’s sole fuel, except under conditions of starvation, when it breaks down protein. The brain is by far the body’s hungriest organ. Although it accounts for just 2 percent of the body’s weight, it requires a staggering 20 percent of its total glucose supplies. This is obtained from dietary carbohydrate, which is transported to the brain via the bloodstream. It consumes roughly 4oz (120g) of glucose (about 420kcal) per day. Because the brain cannot store glucose, it must be readily available at all times via the blood supply. Without oxygen or glucose, the brain can last for only about 10 minutes before irreparable damage occurs. This is why prompt resuscitation is needed in cases of cardiac arrest. PROTECTING THE BRAIN The brain has several defense mechanisms to protect it from damage. The bony skull acts as a box, containing the brain and buffering it against blows. The meninges are three layers of membranes that line the skull, enclosing the brain and providing extra layers of protection between the skull and the brain. Cerebrospinal fluid circulates within the brain, nourishing brain tissue and working as a shock absorber to reduce the impact of knocks. CEREBROSPINAL FLUID FLOW Brain tissue floats in cerebrospinal fluid (CSF) within the skull. CSF absorbs shocks from blows to the brain. It is produced in a series of connected chambers within the brain known as the ventricles, and is renewed four to five times per day. It contains proteins and glucose to nourish brain cells, as well as white blood cells to protect against infection. It moves through the ventricles, propelled by the pulsation of the cerebral arteries. 3 Circulation around spinal cordHelped by vertebral movement, fluid travels downward along the back of the spinal cord, into the central canal, and upward along the front of the cord. 4 Site of reabsorption (arachnoid granulations) After traveling around the brain, the fluid is finally reabsorbed into the bloodstream through tiny arachnoid granulations (projections from the arachnoid layer of the meninges into the sagittal sinus). 2 Direction of flowCSF flows from the lateral ventricles into the third and fourth ventricles. It then continues up the back of the brain, down around the spinal cord, and to the front of the brain, as indicated by the arrows. 1 Site of fluid production (choroid plexus) CSF is produced in the clusters of thin-walled capillaries (the choroid plexus) that line the walls of the ventricles. Third ventricle Fourth ventricle Spinal cord Cerebellum Central canal Lateral ventricle Dura matter Sagittal sinus Skull Dura mater Cerebral vein Skull Blood vesselCerebrum Arachnoid Pia mater THE CIRCLE OF WILLISZThe angiogram above and the illustration to the left show the Circle of Willis, a ring of communicating arteries encircling the base of the brain. It provides the brain with supply routes for glucose and oxygen. If one route becomes blocked, another one compensates for it. THE MENINGES The outermost layer, the dura mater, contains blood vessels; the arachnoid consists of connective tissue; and the pia mater lines the brain’s contours. Anterior inferior cerebellar artery Posterior cerebral artery Optic nerve Anterior spinal artery Anterior communicating artery Anterior cerebral artery Superior cerebellar artery Basilar artery Vertebral artery Labyrinthine artery B R A IN S IZ E , E N E R G Y U S E , A N D P R O T E C T IO N I T H E B R A IN A N D T H E B O D Y Internal carotid artery 4 5 US_044-045_size_weight.indd 45 02/08/18 2:32 PM CIRCLE OF WILLIS The major arteries of the brain can be seen in this MRI scan. They include the Circle of Willis (below center) at the base of the brain, where arteries from the neck meet before branching. US_046-047_blood_flow_dps.indd 46 01/08/18 5:23 PM OXYGEN SUPPLY This arteriograph shows arteries carrying oxygen-rich blood to the brain. The arrangement of the arteries allows blood to be supplied by another route if one of the pathways is blocked. US_046-047_blood_flow_dps.indd 47 01/08/18 5:23 PM BRAINS EVOLVED TO ENABLE ANIMALS TO RESPOND TO ENVIRONMENTAL CHANGES. THE HUMAN BRAIN HAS EVOLVED TO ITS PRESENT COMPLEXITY THROUGH SEVERAL STAGES, MANY OF WHICH ARE COMMON TO ALL ANIMALS. ITS ORIGINS CAN BE SEEN IN THE BRAINS OF OTHER SPECIES, IN WHICH MORE PRIMITIVE STRUCTURES REMAIN. EVOLUTION EVOLUTION OF THE INVERTEBRATE BRAIN All animals have to respond to changes in their internal and external environment in order to survive. To do this, they have evolved cells that are sensitive to stimuli such as light and to vibrations. The sensory cells are, in turn, connected to other cells that can move the organism or change its state in response to the stimulus. This system of interconnected nervous tissue is a crude form of brain. In invertebrates, such as worms, the nervous system is distributed throughout the creature’s body, as a loose network of reactive fibers. Some of these networks contain small masses of nerves, known as ganglia. These are the forerunners of the structures that, in some species, have become the central nervous system or brain. EVOLUTION OF THE VERTEBRATE BRAIN Through the course of evolution, the brain has undergone considerable changes. Compared to the primitive nervous systems of invertebrates, the brain of vertebrates is a well-developed, highly interconnected organ. The central nervous system is connected to the rest of the body by a peripheral nervous system that includes the fibers running to and from the sensory organs. The basic vertebrate brain—also sometimes referred to as the “reptilian brain”— EARTHWORM BRAIN The earthworm has a crude brain, the cerebral ganglion, which is connected to a cord of nervous tissue (the ventral nerve cord) that runs the length of its body. Nerve fibers from the cord extend into each segment, so muscle contraction along the body can be coordinated to produce movement in response to stimuli. consists of the cluster of nuclei that lies just above the brainstem in humans. They include the modules that produce arousal, sensation, and reaction to stimuli. It is unlikely, however, that these nuclei alone are sufficient to produce consciousness. This basic vertebrate brain does not include more advanced features, such as the limbic system or cerebral cortex, which exist only in the brains of mammals. PRIMITIVE NERVOUS SYSTEM The simplest system, as seen in this hydra (a tiny aquatic invertebrate), consists of a loose network of sensory cells with clumps of interconnected cells called ganglia. T H E B R A IN A N D T H E B O D Y I E V O L U T IO N FISH FROG TURTLE Ventral nerve cord Brain Ganglia Oesophagus Cerebellum Optic lobe Cerebrum Pituitary gland Medulla Olfactory bulb KEY TO VERTEBRATE BRAIN AREAS The amphibian brain resembles the fish brain except that the cerebrum is roofed over with nervous tissue. The main function of this region is to perceive smell, as reflected by the large olfactory bulb. The forebrain is much larger than the cerebellum. AMPHIBIANS A fish’s cerebrum receives sensory signals from the sense organs and combines them with information from the internal organs and nerves to guide action. Fish have a large cerebellum in order to coordinate movement and gauge pressure. FISH Modern reptiles show greater development in the basal parts of the forebrain, and the cerebrum is much larger than the optic lobe. The olfactory bulb is large in comparison with the other structures of the brain and is well developed. REPTILES 4 8 US_048-049_comparison.indd 48 02/08/18 2:32 PM MAMMAL BRAINS The mammalian brain comprises a cluster of structures that evolved on top of the basic vertebrate brain, known as the limbic system, and a wrinkled covering called the cortex, which interconnects with the limbic structures beneath. The limbic system is the part of the brain that produces emotions. These are responses to stimuli that go beyond the basic “grab” or “avoid” reactions in the vertebrate brain, and produce subtle and complex actions that are not always predictable. The limbic system also contains structures that encode experiences as memories, to be recalled for use in guiding future actions. The emotional and memory faculties greatly increase the range and complexity of behavior that a mammal displays, because it is not governed purely by instinct. HOMINID BRAINS The brains of hominids (modern humans and their ancestors) underwent a surge of evolutionary changes that left them, in some ways, distinctly different even from their near relatives, such as chimpanzees and gorillas. The main distinction between human and other mammalian brains is the size and density of the cortex, and particularly of the frontal lobe, which is responsible for complex thought, conscious judgement, and self-reflection. No one knows why the human brain evolved as it did—it may have been due to some change in diet forced by the environment, or the product of living in groups (see p.138) that depended on close interdependence for survival. BRAIN SIZE AND SHAPE One striking aspect of mammalian brain evolution is the development of the cortex. This outer layer has evolved to serve the particular needs of each species, and therefore varies dramatically between one animal and another. A few mammals, such as humans, elephants, and dolphins, have a disproportionately large cortex compared to most mammals. DOES SIZE MATTER? The growth of the human brain over the course of evolution is thought to be the reason why we are so dominant. However, size is not the only factor that matters for intelligence or survival—the way brains are wired up may be more important. Neanderthals had bigger brains than humans, but were less innovative and were finally superseded by other hominids. THRUSH CAT HUMAN ELEPHANT HUMAN DOLPHIN CAT WOLF 1400 1600 BRAIN VOLUME (CUBIC CENTIMETERS) NEANDERTHAL SKULL HOMO NEANDERTHALENSIS MODERN HUMANS 0 200 400 600 800 1000 1200 CHIMPANZEE GORILLA HOMO HABILIS HOMO ERECTUS E V O L U T IO N I T H E B R A IN A N D T H E B O D Y Birds’ brains are similar to those of reptiles except that the cerebellum is highly developed to control balance and position in flight. Despite the size of the olfactory bulb, most birds have a poor sense of smell, with some exceptions, such as the kiwi. BIRDS In mammals, the cerebellum is relatively small compared to the forebrain. The cerebrum is covered in wrinkled cortex; these wrinkles allow a greater volume of cortex to fit into the skull, compared to the smooth surface of the reptilian brain. MAMMALS The human brain is completely dominated by the cerebrum, and the cortex is intricately folded to allow the maximum amount to be contained in the skull. The cerebellum remains large and active, however, to enable complex motor activity. MAN 4 9 US_048-049_comparison.indd 49 01/08/18 5:23 PM BRAIN ANATOMY IS HIDDEN, SECRET, AND MORE COMPLEX THAN ANY OTHER PART OF THE BODY. THE BASIC BUILDING BLOCK OF THE BRAIN IS THE CELL. SIGNALING CELLS KNOWN AS NEURONS FORM LARGER STRUCTURES CALLED NUCLEI THAT CARRY OUT PARTICULAR FUNCTIONS. THEY ALSO CLUSTER TOGETHER TO FORM THE THICK, LAMINATED SHEET OF GRAY MATTER FORMING THE COVERING OF THE BRAIN CALLED THE CORTEX. DEEP FISSURES IN ITS SURFACE DIVIDE THE BRAIN INTO TWO HALVES (THE HEMISPHERES), EACH WITH FIVE LOBES. THESE MAJOR DIVISIONS “SPECIALIZE” IN DIFFERENT TASKS, BUT ALSO INTERCONNECT AND INTERACT. BRAIN ANATOMY US_050-051_brainanat_op.indd 50 01/08/18 5:23 PM BRAIN ANATOMY US_050-051_brainanat_op.indd 51 01/08/18 5:23 PM THE BRAIN HAS A COMPLEX AND MANY-LAYERED ANATOMY. PEELING BACK THE DOMINANT CEREBRAL HEMISPHERES REVEALS A FURTHER SET OF STRUCTURES WITHIN. SOME ARE DISCRETE MASSES, SUCH AS THE CEREBELLUM AND THALAMUS, WHILE OTHERS ARE ZONES OF NERVE FIBERS OR NERVE CELLS WITHIN LARGER STRUCTURES, DISCERNIBLE ONLY BY MICROSCOPIC EXAMINATION. BRAIN STRUCTURES Caudate nucleus Putamen Corpus callosum Amygdala Cerebellum Hippocampus Right hemisphere EXPLODED HEAD A whole head “exploded” sideways reveals the main brain regions or divisions. The central brainstem stands up like a fist on an arm, and the cerebrum wraps over and around it, dominating it both physically and mentally. The next largest structure after the cerebrum is the cerebellum at the lower rear, comprising about ten percent of the brain’s total volume. In common with standard anatomical terminology, right and left refer to the owner rather than the viewer. So here the right hemisphere of the cerebrum is on the left of the picture. B R A IN A N A T O M Y US_052-053_brain_anatomy.indd 52 02/08/18 2:32 PM THE BRAIN HIERARCHY The brain’s major parts can be classified or categorized in several ways. In all of these systems, the dominant part is the cerebrum, the large pinky-gray wrinkled structure that forms more than three-quarters of the brain’s total volume. The cerebrum is divided into left and right hemispheres, which are linked by a “bridge” of nerve fibers, the corpus callosum. The cerebrum, which includes the hippocampus and amygdala, is also known as the telencephalon. Together with the parts it wraps around—the thalamus, hypothalamus, and associated parts, collectively known as the diencephalon—it comprises the major brain “division” known as the forebrain (prosencephalon). Below the forebrain is the midbrain (mesencephalon), a small division that includes groups of nerve-cell bodies known as nuclei, such as the basal ganglia. Below the midbrain is the hindbrain (rhombencephalon), with the pons as its uppermost part, and beneath it the cerebellum and the medulla, which tapers to merge with the spinal cord. B R A IN A N A T O M Y Hypothalamus Fornix Thalamus Interior globus pallidus Exterior globus pallidus Optic chiasm Trigeminal nerve Subthalamic nucleus Pituitary gland Olive (rounded protrusion that contains the olivary nuclei) Pons Superior colliculus Pyramid (anterior medulla) Medulla Cervical spinal cord (in neck) Thoracic spinal cord (in chest) Geniculate nucleus Midbrain Maxilla (upper jaw bone of skull) Nasal cavity Sphenoid bone of skull Foramen magnum (hole for spinal cord) Occipital bone of skull Left hemisphere Cervical vertebra (neck backbone) Spinal nerve US_052-053_brain_anatomy.indd 53 01/08/18 5:23 PM SCALP SKIN The skin of the scalp has only a thin underlying layer of subcutaneous fat and the hard skull is just beneath, so it wounds relatively easily and bleeds copiously. SCALP NERVES Many small peripheral nerves branch through and under the scalp skin from cranial nerves II, III, and V. Even faint contact registers, allowing us to react quickly and avoid injury. SKULL The upper domed part of the skull, called the neurocranium, forms a “braincase” to shield against knocks and jolts. This function is aided by the meninges (see p.56). Internal globus pallidus External globus pallidus Sulcus Gyrus Putamen Caudate nucleus Right thalamus Right cerebral hemisphere FRONTAL BONE The neurocranium is composed of eight bones. Most prominent is the frontal bone under the forehead. The left and right parietals are behind it, the occipital below them at the lower rear, and the two temporals on the lower sides. The sphenoid and ethmoid bones are at the lower front, behind the nose area. FACIAL BONES Complicated in shape, the facial bones have gaps (foramina) in them. Some allow cranial nerves to pass from the brain within the neurocranium, out to the nasal epithelium in the nose cavity, the eyes in their sockets, the inner ear, and other sensory parts. Blood vessels have similar sets of skull foramina. B R A IN A N A T O M Y 5 4 US_054-055_exploded_brain.indd 54 01/08/18 5:23 PM LEFT AND RIGHT HEMISPHERES An overhead view of the “exploded” brain shows how the two cerebral hemispheres can be neatly separated by cutting through the corpus callosum. Many other brain structures are symmetrically paired in this way, such as the thalamus, which is sometimes described as “two hen’s eggs sitting side by side.” The cerebellum at the lower rear of the brain is accommodated within a bowl-like cavity of the skull known as the posterior cranial fossa. The cranial nerves (numbered I to XII, see p.43) enter the brain directly rather than connecting to the spinal cord. CEREBELLUM This name means “little brain,” referring to the pattern of grooves and bulges on the cerebellar surface, which reflects the external appearance of the cerebrum. The cerebellum is connected to the brainstem immediately in front of it by three pairs of thick, short, stalklike extensions, called the cerebellar peduncles. CEREBRAL CORTEX The thin grayish covering of each cerebral hemisphere is called the cerebral cortex. It has a characteristic pattern of bulges (gyri), shallower grooves (sulci), and deeper ones (fissures). Fornix Left thalamus Pineal gland Mamillary body Hypothalamus Midbrain Subthalamic nucleus Left olfactory tract (cranial nerve I) B R A IN A N A T O M Y 5 5 US_054-055_exploded_brain.indd 55 01/08/18 5:23 PM B R A IN A N A T O M Y I B R A IN Z O N E S A N D P A R T IT IO N S Hypothalamus Situated under the thalamus, as its name implies (“hypo” means “under”), the sugar-cube-sized hypothalamus has many important functions, including temperature control and basic behavioral drives Pituitary gland “Master gland” of hormonal or endocrine system hangs by a stalk from hypothalamus above Cerebellum Responsible for balance and posture Neck vertebra Spinal cord Thalamus Processes and sends on sensory information to higher brain areas Pons “Crossroads” area consisting mainly of nerve fibers Corpus callosum Main link between left and right cerebral hemispheres is a highway of more than 200 million nerve fibers Medulla Regulates vital functions such as heartbeat and respiration SLICED DOWN THE MIDDLE A medial sagittal section (a cut through the brain from front to rear, exactly in the middle or center line between the eyes) shows the sliced corpus callosum and brainstem. The left cerebral hemisphere and thalamus are off-center, so they remain unsectioned. Superior sagittal sinus Around the brain’s midline is a shallow groove containing blood, which is part of the venous return to heart Dura mater and arachnoid The outer two meninges are the tough, strong dura mater attached to the inside of the skull, and the blood-rich arachnoid Subarachnoid space This gap between the arachnoid and pia mater is filled with cushioning cerebrospinal fluid Pia mater Innermost meninx Specific names are given to various sections or slices of the brain, which show different views of the internal parts. For example, a sagittal section that is not medial (down the middle), misses the corpus callosum and cuts down through a cerebral hemisphere to reveal its intricate pattern of surface folds and grooves. S E C T IO N IN G T H E B R A IN HORIZONTAL CORONAL SAGITTAL MEDIAL Skull Scalp 5 6 US_056-057_brain_anatomy.indd 56 01/08/18 5:23 PM THE BRAIN’S PHYSICAL STRUCTURE BROADLY REFLECTS ITS MENTAL ORGANIZATION. IN GENERAL, HIGHER MENTAL PROCESSES OCCUR IN THE UPPER REGIONS, WHILE THE BRAIN’S LOWER REGIONS TAKE CARE OF BASIC LIFE SUPPORT. BRAIN ZONES AND PARTITIONS THE HOLLOW BRAIN The brain has an internal system of chambers (ventricles), which are filled with a liquid—cerebrospinal fluid (CSF)—produced by the ventricle linings. The upper two chambers are the left and right lateral ventricles, one in each cerebral hemisphere, with hornlike forward- and side-facing projections. Small openings connect them to the third ventricle in the midbrain, which in turn links to the fourth ventricle in the pons and medulla. CSF flows slowly and continuously through the ventricles, then out via small openings into the subarachnoid space around the brain and the spinal cord. VERTICAL ORGANIZATION The uppermost brain region, the cerebral cortex, is mostly involved in conscious sensations, abstract thought processes, reasoning, planning, working memory, and similar higher mental processes. The limbic areas (see pp.64-65) on the brain’s innermost sides, around the brainstem, deal largely with more emotional and instinctive behaviors and reactions, as well as long-term memory. The thalamus is a preprocessing and relay center, primarily for sensory information coming from lower in the brainstem, bound for the cerebral hemispheres above. Moving down the brainstem into the